The age dependent levels of serum ALP isoenzymes and the diagnostic significance of corticosteroid-induced ALP during long-term glucocorticoid treatment

Three isoenzymes of total alkaline phosphatase (TALP) are known in canine serum: Bone alkaline phosphatase (BALP), liver alkaline phosphatase (LALP) and corticosteroid-induced alkaline phosphatase (CALF). Using an assay developed by combining selective precipitation of BALP by wheat germ lectin (WGA) and an automated levamisole inhibition method for quantifying CALF, age-related reference ranges of the isoenzymes in 75 canine serum samples were investigated. BALP comprised 96, 38 and 26% of TALP in young, middle aged and old dogs, respectively, and CALF was respectively 12, 11 and 27% of TALP. LALP was less than 10% in the young but represented more than 50% of TALP in middle aged and old dogs. Furthermore, the significance of monitoring LALP and CALF and their relationship to hepatopathy in dogs receiving long term prednisolone therapy was assessed. In this study, TALP increased in all dogs receiving prednisolone. But only LALP wits responsible in dogs with minor vacuolization of the liver, while in severely degenerated cases both LALP and CALF increased. It is concluded that a high TALP due solely to LALP, rather than LALP and CALF represents lesser liver pathologic involvement. Monitoring the 2 isoenzymes has greater significancy in assessing the level of liver damage than relying on an increased TALP value alone. Quantifying the individual isoenzymes may further be useful in assessing the clinical significance of these isoenzymes in various conditions that result in elevated TALP values.